Non-martensitic structures of improved ductility and cold working properties and improved machinability in ferrous metals, in particular in carbon steel, have heretofore been obtained by cooling the austenitized ferrous metal either (1) in one of the usual quenchants, such as water, aqueous solutions, quenching oils or molten salt baths in order to obtain mainly martensitic microstructures, followed by subsequent tempering in order to transform these comparatively hard and brittle structures into more ductile or machinable tempering structures, a process which is known as "quenching and tempering", or (2) in a molten lead bath or salt at a temperature between about 500.degree. and 650.degree. C. in order to obtain directly fine stripped pearlite, a process which is generally known as "patenting".
The conventional and well known aqueous-base quenchants used for quenching to produce non-martensitic microstructures by the "quenching and tempering" process may be used to produce uniform non-martensitic structures. However, such quenching and tempering process, which consists of three stages, namely austenitizing, quenching and tempering, generally produces microstructures, such as spherodite, which are unsuitable for subsequent drawing and certain machining operations. The reason for this is that these quenchants do not develop a vapor envelope at the surface of the metal being quenched which is sufficiently stable and uniform to provide the necessary slow and uniform cooling to obtain the desired fine striped pearlite structure free of martensite.
Accordingly, resort has been made to the use of molten lead and salt baths to obtain the necessary reduced quenching rates which provide the desired non-martensitic structures. However, molten lead and salt baths, used mainly in the patenting process, constitute hazards in the form of destructive fires, burns of the skin of operating personnel, and air and water pollution.
U.S. Pat. No. 2,994,328 discloses the patenting of steel rod immediately after it has been hot rolled by continuously passing the rod through a series of water cooling stands. The patenting conditions are controlled by adjusting the flow of water to each of the water cooling stands. This method requires the use of a number of water cooling stands, is relatively complicated and produces undesirable results, in particular, the formation of coarse pearlite, which has only limited ductility and cold working properties.
U.S. Pat. No. 3,231,432 discloses a patenting process in which austenitized steel wire is cooled by means of forced gas. Unfortunately, it is considerably more difficult to control the rate of cooling in this manner than by use of a liquid quenchant. In addition, such process can be employed only with a limited number of alloys and with rod of a limited range of diameters.
U.S. Pat. No. 3,669,762 proposes the patenting of hot rolled carbon steel rod by quenching in hot water which is supposed to generate a stable steam envelope around the rod. The water quenchant, which may contain 0.1 to 2.0%, by weight, of a surface active agent, is at a temperature of 45.degree. C. to 100.degree. C., preferably 70.degree. C. to 100.degree. C., and should not vary by more than 5.degree. C. from the selected temperature. The process is limited to patenting of steel rod which is free of rough or coarse scale on its surface, because such type surface would cause the steam film to collapse resulting in the formation of areas of martensite in the wire. The presence of such areas is extremely undesirable, since breaking of the wire during subsequent coiling or drawing process takes place in such areas.
In quenching parts formed of alloy steels to obtain a martensitic structure various problems have arisen using available quenchants. Not infrequently the quenched parts have been cracked and/or distorted. Accordingly, there has been a need for quenchant solutions which, by adjusting various parameters thereof, make possible a wide selection of cooling rates whereby parts having the desired microstructure free of defects can be obtained.
It is a primary object of the present invention to provide a new and useful process for the cooling of austenitized ferrous metal parts with the aim to produce therein non-martensitic microstructures having improved ductility and cold working properties, and improved machinability, such as fine striped pearlite without post quenching heat treatments.
Another object is to provide a process in which parts of austenitized ferrous metal are quenched in comparatively cool aqueous polyacrylate salt solutions whose parameters are such as to provide uniform, low cooling rates by reason of the development of a stable and uniform envelope of steam at the surface of the parts, whether or not such surfaces are rough or are covered with rough or coarse scale.
Still another object of the invention is to provide a quenching process in which cooling rates can be varied widely, but which is not temperature sensitive, whereby substantial fluctuations in the temperature of the quenching solution do not impair the uniformity of the quality of the quenched parts.
A still further objective is to provide a process for quenching parts formed of alloy steels to obtain therein martensitic structures, the quenched parts being free of undesirable cracks and distortion.
Another object is to provide a quenching process in which the liquid medium of the quenching bath is non-flammable, non-explosive, non-toxic, and non-pollutive, whereby injury to operating personnel and environmental pollution are avoided.